In side-on collision airbag devices, structures have been disclosed wherein the airbag interior is partitioned by a seam into a lower chamber and an upper chamber, and a gas distributor (diffuser) is provided to a separated portion between a rear end of the seam and a rear edge of the airbag (see Japanese Patent Application Laid-Open (JP-A) No. 2004-268682). This gas distributor is configured in a tube shape from heat-resistant fabric, and is disposed such that its length direction is aligned with the top-bottom direction. A lower portion of the gas distributor is configured as a non-return valve so as to prevent gas from flowing from the bottom chamber towards the top chamber.